The invention relates to a method for air quantity control in a closed pneumatic spring system in a vehicle, in which method a compressed air pump supplies at least two pneumatic springs and/or a compressed air store on demand with an air quantity of such a magnitude that a high air pressure prevails in the respective pneumatic spring and/or in the compressed air store such that a vehicle body which is supported on the pneumatic spring is at a desired distance from the underlying surface or from the vehicle axle.
It is generally known that pneumatic springs in motor vehicles can be utilized to adjust the height of the vehicle body relative to the underlying surface and to maintain a selected distance to the underlying surface under different vehicle loading. For this purpose, pneumatic springs of said type are arranged at least one vehicle axle, between the latter and the body, and are connected to a compressed air source via compressed air lines. The respective pneumatic springs are adjusted on the basis of suitable items of sensor information which are processed by an open-loop and closed-loop control unit to form adjustment signals for valves arranged in the compressed air lines.
It is known from EP 0 779 167 B1 with regard to a level controlling device that the air quantity in the pneumatic spring bellows determines the height of the vehicle body above the vehicle axle, so that said distance can be changed by adjusting said air quantity. Since the pressure, volume and temperature of a gas are linked to one another in a known way by the gas laws, a change in the air quantity in a pneumatic spring bellows for a constant volume of the bellows results in a change in the air pressure in said bellows, which effects a change in level. In the level controlling device known from said document, it is provided inter alia that the air pressure measured in the pneumatic spring bellows for a constant vehicle level can be utilized as a measure of the loading of the vehicle, the mean value of the measured pressure values of the pneumatic springs of a vehicle axle preferably being utilized.
Level control systems of said type are to some extent designed as closed systems in which a compressed air store can be pressurized with a nominal air pressure by means of a compressed air pump. In closed compressed air systems of said type, the pneumatic springs are conventionally supplied with compressed air from the compressed air reservoir of the compressed air store. The compressed air pump fills the compressed air store in a discontinuous fashion whenever the system air quantity falls below a minimum limit value.
Overall, a closed compressed air system of said type contains a certain system air quantity which is to be considered constant during uninterrupted operation. Changes in the system air quantity are to be attributed, for example, to leaks in the system and/or to temperature changes. If the system air quantity falls below a predetermined nominal value, this can generally also be identified as a pressure loss as a result of which the compressed air pump is activated until the nominal value of the system air quantity is again enclosed in the compressed air system.
In order that the described controlling process is not triggered in the event of every small deviation from the nominal value of the system air quantity, controlling processes of said type provide an air quantity tolerance band whose upper and lower limit values, like the nominal value of the system air quantity according to the prior art, are predetermined constant variables for all level controlling and vehicle loading states.
Against this background, a method for controlling the pressure in the compressed air store in a closed level control system is known from DE 101 22 567 C1. The level control system there has a compressor and a compressed air store which can be filled with air from the atmosphere and can be discharged to the atmosphere. At least one pneumatic spring is additionally provided which is connected to the compressed air store via the compressor in such a way that compressed air can be conveyed from the pneumatic spring into the compressed air store and vice versa.
According to said document, a method for controlling the store pressure of the compressed air store should be used in the described level control system, said method ensuring that the store pressure is in a certain range and that the compressed air store is not unnecessarily filled with compressed air from the atmosphere or discharged to the atmosphere.
In a certain state of the motor vehicle, the store pressure should preferably be in a certain range in order to ensure fast lifting and lowering of the vehicle body. In this context, it is provided that the store pressure of the compressed air store is controlled indirectly by means of the determination of the air quantity in the level control system. Here, the compressed air store is filled with air from the atmosphere when the air quantity is below a limit, and is discharged to the atmosphere when the air quantity is above an upper limit. Said control is carried out in such a way that the actual air quantity in the system after filling or discharging is in a working range between the upper and lower limits.
According to said document, it is preferably also provided that the air quantity is always kept in a certain working range and, as a result, the store pressure is also in a certain pressure range in the certain state of the vehicle. Here, the working range for the air quantity is preferably selected such that, in a certain normal state of the vehicle, the store pressure is in a range in which both fast lifting and fast lowering of the vehicle body is possible.
Although a controlling method of said type is functional, it has proven to be disadvantageous that, for all level and loading states, comparatively large differences can be detected in the required controlling times for different level and loading states, in particular as a result of the constant nominal air quantity value and as a result of the fixed air quantity tolerance band. In addition, there is a trade-off in the control of the extreme states “controlled lowering at empty load” and “controlled lifting at full load”. Finally, controlling processes which operate in such a way require a comparatively large compressed air store, which has an adverse effect on the production costs of a generic level control system.
Against this background, the invention is based on the problem of proposing a method for the open-loop and closed-loop control of a level control system, with which method constant controlling speeds are possible in all vehicle loading states. Said method should additionally be suitable for allowing the smallest possible pressure store to be used.